This week we learned about how the anthocyanin and flavonol pigments in red cabbage act as indicators. The anthocyanins turn reddish pink under acidic conditions and blue under more basic conditions, whereas the flavonols were colorless under acidic conditions and yellow under basic conditions. This resulted in pinkish colors under acidic conditions (reddish pink + colorless pigments) and green colors under basic conditions (blue + yellow). One of the materials we got to experiment with was household bleach, which, like most cleaning agents, is basic. However, we all noticed an interesting effect: bleach did turn the cabbage paper green.
More accurately, the bleach turned the paper green at first. Then the colors quickly faded away, resulting in pure white. I decided to do a little research this week on why this happens: how does bleach work?
Household bleach is commonly made up of sodium hypochlorite (NaOCl) in water. The sodium hypochlorite tends to dissociate into sodium ions (Na+) and hypochlorite ions (–OCl), and the hypochlorite ion is the active ingredient. It is able to produce a reaction known as oxidation, which can break some key bonds in colorful molecules. When these bonds are broken, the molecule no longer reflects light in the same way, and the substance it makes up becomes colorless.
How does this play out for red cabbage? The anthocyanin and flavonol structures under basic conditions are shown below.
The color of these compounds comes from the highly conjugated system of rings they contain (the places where there are lots of alternating single and double bonds). The wavelength of light that is reflected/absorbed depends on the electrons and how they form bonds in the molecules; these types of bonds tend to produce very vibrant colors.
From what I can tell, the oxidation reaction from the hypochlorite ion breaks open the rings in the upper right corner of the molecules. This disrupts the conjugated system of bonds, destroying their colorful effect and leaving our paper white rather than green.
Yay! I am so glad you researched this after we discussed this in our group. The relationship between how the bonds are formed and wavelength is an interesting connection between chemistry and physics.
I did not initially process that the bleach’s effect on the red cabbage paper happened in two stages. In the description above, the explanation of different lengths of wavelengths creating different visual effects fuses together the past two classes we have learned of how chemical compositions are translated into what we can see.